1. Technical Field
Processes for recovering and/or producing para-xylene from reformate splitter bottoms and toluene column bottoms are disclosed. More specifically, low temperature processes for recovering and/or producing para-xylene from reformate splitter bottoms and toluene column bottoms are disclosed and heat exchanger networks for the low temperature processes are also disclosed.
2. Description of the Related Art
The xylene isomers, meta-xylene, ortho-xylene and, in particular, para-xylene, are important chemical intermediates. Ortho-xylene is oxidized to make phthalic anhydride which is used to make phthalate based plasticizers among other things. Meta-xylene is oxidized to make isophthalic acid which is used in unsaturated polyester resins.
However, para-xylene has by far the largest market of the three isomers. The largest use of para-xylene is in its oxidation to make terephthalic acid. Terephthalic acid, in turn, is used to make polymers such as polytrimethyleneterephthalate, polybutyleneterephthalate (PBT), and polyethyleneterephthalate (PET). PET is made via condensation polymerization of terephthalic acid with ethylene glycol.
PET is one of the largest volume polymers in the world. It is used to make PET plastics, e.g., two liter beverage bottles. PET is also used to make polyester fiber which, in turn, is used to make clothes and other fabrics Polyester fiber is used both as a homofiber, as well as a blended fiber, such as a blend with cotton. Given the large market for PET plastics and fibers, there is a substantial demand for high purity para-xylene. Further, the demand for para-xylene is several times larger than the demand for ortho and meta-xylene. The demand for para-xylene is also larger than the amount of para-xylene in the xylenes recovered as a by-product from reformate processes, such as the xylenes recovered from catalytic reformers and from pygas (i.e., high temperature clacking to make light olefins). Because the demand for para-xylene is so much larger than the demand for the other xylene isomers and is larger even than the supply of para-xylene in xylenes recovered as a by-product of other processes, it has been found that isomerization of xylene isomers is desirable to increase the amount of para-xylene production.
Para-xylene is typically produced by reforming or aromatizing a naphtha feed in a reformer, for example, a continuous catalytic reformer, and then separating by distillation a C8 aromatics rich fraction from the reformer effluent. The C8 fraction includes neat equilibrium amounts of ethylbenzene and the three xylene isomers, namely, para-, meta- and ortho-xylene. The para-xylene in this C8 aromatics fraction can then be separated using an adsorption process such as a simulated moving bed (SMB) adsorption process Downstream of an adsorption unit, the para-xylene depleted C8 aromatics stream is typically further processed by passing it over a xylene isomerization catalyst in a xylenes isomerization unit. The resulting C8 aromatics stream, now with an approximately equilibrium concentration of xylenes, i.e., a higher concentration of para-xylene (˜22 wt %), is recycled to the para-xylene separation process. Thus, the adsorption and isomerization processes are typically employed together; in a loop
The xylene isomerization unit can serve at least two functions. First, it re-equilibrates the xylenes portion of the stream, bringing the para-xylene concentration up to the equilibrium concentration of 22 wt %. Thus, in effect, it is creating para-xylene from the other xylene isomers. Second, combination of ethylbenzene isomerization catalyst and ethylbenzene dealkylation catalyst in the isomerization process converts ethylbenzene into additional mixed xylenes as well as converts ethylbenzene to a benzene co-product. Since ethylbenzene boils in the same range as the xylene isomers, it is not economic to recover/remove the ethylbenzene by distillation, hence it is included in the C8 aromatics fraction that is fed to the para-xylene SMB adsorption process. It is highly desirable to remove as much ethylbenzene as possible pet pass so that it does not accumulate in the recycle loop.
Thus, a critical function of the isomerization unit is to convert the ethylbenzene to xylene isomers and benzene by either isomerization and dealkylation, or other means for removing ethylbenzene, depending upon on the type of isomerization process employed. This function is critical because the boiling points of the four C8 aromatics at issue fall within a very narrow 8° C. range, from about 136° C. to about 144° C. (see Table I).
TABLE IC8 CompoundBoiling Point (° C.)Freezing Point (° C.)ethylbenzene136−95para-xylene13813meta-xylene139−48ortho-xylene144−25
As shown above, the boiling points of para-xylene and ethylbenzene are about 2° C. apart. The boiling points of para-xylene and meta-xylene are only about 1° C. apart. As a result, fractional distillation would be impractical is it would require large equipment, significant energy consumption, and/or substantial recycles to provide effective and satisfactory xylene separations.
In processing reformate to extract and then a separate para-xylene product stream, the reformate is first subjected to a separation to remove C7 and lighter material and then fractionated to form a C8 aromatic concentrated material, or xylene enriched stream, which will also include ethylbenzene. This stream is then subjected to an adsorption process to produce an essentially pure para-xylene product, after the desorbent is extracted and toluene removed by fractionation. As noted above, the raffinate from the adsorption process is isomerized to produce para-xylene from the C8 isomers in the raffinate and the product of isomerization is recycled to the adsorption process for removing the newly formed para-xylene. Usually the isomerized raffinate is fractionated before it is recycled to the initial fractionation process upstream of the adsorption unit.
Currently, the adsorption process for separating para-xylene from the other xylene isomers and ethylbenzene is carried out at a temperature of about 177° C. (350° F.). Recent experimental work indicates that operating the adsorption section at a lower temperature improves the SMB adsorption unit productivity Specifically, for a constant cycle time, adsorbent capacity increases as the adsorption temperature decreases. Thus, new processes are required to accommodate modifications of the operating temperature adsorption processes